1. Field of the Invention
The present invention relates to a semiconductor device, and particularly relates to a semiconductor device which achieves reduction of both a reverse recovery time and a forward voltage.
2. Description of the Related Art
A fast recovery diode (FRD, hereinafter referred to as a Si-FRD) using a silicon semiconductor substrate is known as a pn junction diode with improved switching time. This technology is described for instance in Japanese Patent Application Publication No. Hei 10-335679 (page 20, FIG. 39).
Meanwhile, a Schottky barrier diode (SBD, hereinafter referred to as a Si-SBD) using a silicon semiconductor substrate is known to have a short reverse recovery time which is a characteristic of the device itself.
Moreover, an SBD using a silicon carbide substrate (hereinafter referred to as a SiC-SBD) is in the process of marketing as a next-generation device.
FIG. 6 is a diagram showing characteristics of a forward voltage VF and a reverse recovery time trr in a case of IF=10 A in a Si-FRD and a SiC-SBD.
The reverse recovery time trr needs to be reduced in the Si-FRD. The reverse recovery time is a time required until an amount of charge Qrr accumulated in the device is emitted at the time of turn-off when a forward bias is turned into a reverse bias. However, reducing the reverse recovery time trr causes a problem of increasing the forward voltage VF (see a dotted line (trade-off line) in FIG. 6).
To be more precise, the characteristics of the forward voltage VF and the reverse recovery time trr (hereinafter referred to as VF-trr characteristics) have a trade-off relationship as shown in FIG. 6. In the current situation, either the forward voltage VF characteristic or the reverse recovery time trr characteristic is sacrificed according to the usage. Meanwhile, the Si-SBD has a short reverse recovery time trr, but requires a very high forward voltage VF when being produced as a device having a breakdown voltage of, for example, 600 V. Accordingly, the Si-SBD has the same problem with the Si-FRD.
In contrast, the SiC-SBD has a low forward voltage VF and a short reverse recovery time trr, and thus the VF-trr characteristics can be improved. However, the SiC-SBD as a completed wafer costs 20 times or more than the Si-FRD, and thus is very expensive.